Xylylenediamine (bis(aminomethyl)benzene) is a useful starting material, for example for the synthesis of polyamides, epoxy hardeners or as an intermediate for preparing isocyanates.
The expression “xylylenediamine” (XDA) comprises the three isomers ortho-xylylenediamine, meta-xylylenediamine (MXDA) and para-xylylenediamine.
Crude xylylenediamine can be prepared, for example, by ammoxidizing xylene and subsequently hydrogenating the resulting phthalonitrile. Possible preparation processes are described, for example, in the applications mentioned below.
U.S. Pat. No. 4,482,741 (UOP Inc.) describes the hydrogenation of phthalonitrile (PN) in the presence of ammonia, a specific catalyst and XDA as a solvent.
EP-A2-1 193 247 and EP-A1-1 279 661 (both Mitsubishi Gas Chem. Comp.) respectively relate to a process for purifying isophthalonitrile (IPN) and to a process for preparing pure XDA.
EP-A2-1 193 244 (Mitsubishi Gas Chem. Comp.) describes a process for preparing XDA by hydrogenating phthalonitrile which is synthesized in a preceding stage by ammoxidation of xylene, the vaporous product of the ammoxidation stage being contacted directly with a liquid organic solvent (quench) and the resulting quench solution or suspension being fed to the hydrogenation.
The purification of the crude XDA obtained is also described in the following patent applications:    JP 2002-088032 (Mitsubishi Gas Chem. Co, Inc.) describes a method for purifying XDA by feeding an inert gas to a condenser, the column top pressure being kept at 53 mbar or lower and the outlet temperature of the condenser being kept at 110° C. or lower. The inert gas strips out components which have a lower boiling point than XDA, so that they are not condensed completely, if at all, but rather are drawn off via the top with the inert gas. A high condensation temperature promotes the effect. However, the burden on the vacuum machine is increased by the inert gas and the components stripped can be condensed out of the inert gas only with difficulty, so that what remains is essentially incineration in a muffle or flare. According to claim 2, inert gas is added as in claim 1 and the bottom temperature is restricted to 180° C. This either gives rise to a large product loss via the bottom or requires a very low vacuum, which, though, is difficult and complicated to generate owing to the inert gas addition.    U.S. Pat. No. 3,647,054 (Japan Gas Chemical) describes the purification of the crude xylylenediamine by adding an alkaline component and subsequent heating in order to hydrolyze residues of unhydrogenated or incompletely hydrogenated phthalonitrile or cyanobenzylamine to subsequently obtain pure xylylenediamine by distillation. Otherwise, phthalonitrile and cyanobenzylamine can barely be separated from xylylenediamine by distillation. The conditions for the hydrolysis reaction are described. Regarding the purifying distillation, the examples mention merely that operation is effected at reduced pressure. U.S. Pat. No. 3,647,054 does not discuss dissociation or decomposition products which are formed at relatively high temperature from xylylenediamine in the distillation and contaminate the product.    SU-A-322 322 of Jan. 26, 1968 (Galperin et al.) describes the purification of crude xylylenediamine which comprises nitrites as secondary components by hydrolyzing the secondary components at 200° C. in the presence of alkali or ammonia. The purifying distillation is not described in detail. There is no discussion of the problems of product decomposition or elimination of ammonia at relatively high temperature.    JP-B-700 14 777 (Japan Gas Chem. Co) describes the purification of xylylenediamine by adding from 0.5 to 5% by weight of an alkaline substance to the crude xylylenediamine based on unreacted phthalonitrile. The crude xylylenediamine was obtained by hydrogenating phthalonitrile in a homogeneous liquid phase of liquid ammonia and organic solvent. Subsequently, the mixture is distilled.    JP-A-2003 026638 (Mitsubishi Gas Chem. Co, Inc.) describes a complicated extraction process with water and aromatic or saturated hydrocarbons for XDA purification. To this end, phthalonitrile is prepared by ammoxidizing xylene, and the reaction gas is quenched with solvent, admixed with ammonia and hydrogenated. Once ammonia and solvent have been removed, crude xylylenediamine is obtained. Water and at least one aromatic or saturated solvent are added. After phase separation, pure xylylenediamine is obtained with the aqueous phase, from which pure xylylenediamine is obtained by distillation.
In order to obtain pure XDA, some of the abovementioned purifications describe complicated processes with which XDA is, for example, purified by extraction, or assistants are added which are intended to ensure the required high purity of the XDA. The addition of assistants is intended to result in hydrolysis of corresponding nitrile which is still present as an impurity in the crude xylylenediamine after the hydrogenation as a result of incomplete reaction.
The processes described are complex in apparatus terms and as a result of the handling of large solvent streams and/or additional components.
The two German patent applications 102005003315.6 of Jan. 24, 2005 and 102005008929.1 of Feb. 24, 2005 (both BASF AG) relate to low-pressure PN hydrogenation in the presence of Raney catalysts.
WO-A-05/028417, WO-A-05/026102, WO-A-05/026103, WO-A-05/026104, WO-A-05/026100, WO-A-05/026101, WO-A-05/026098, WO-A-05/026099 and the two German patent applications 102005036222.2 of Aug. 2, 2005 and 102005045806.8 of Sep. 24, 2005 (all BASF AG) each relate to processes for preparing XDA. All of these processes afford crude xylylenediamine which subsequently has to be purified further.
For example, WO-A-05/028417 teaches on page 8 that the removal of relatively low-and relatively high-boiling by-products can also be effected in a side draw column or dividing wall column, in which case pure xylylenediamine is obtained via a liquid or gaseous side draw. Owing to reactions in the distillation bottoms of crude XDA at high temperatures, it is possible in this way to obtain XDA in good yield only when the distillation is effected at very low pressure in order to restrict the bottom temperature. This is relatively complicated in apparatus terms. In order to obtain pure XDA in the side draw, a product loss via the bottom has to be accepted in the case of vacuum which is easier to realize on the industrial scale. When the yield is increased, the rising bottom temperature leads to product decomposition, in which case the decomposition products, for example methylbenzylamine, contaminate the XDA withdrawn in the side draw. It is thus possible either to obtain pure XDA with a relatively low yield or contaminated XDA with a higher yield.